1. Field of the Invention
The preset application relates to electromagnetic flowmeters. Electromagnetic flowmeters are used in a variety of applications. As is well known, they operate by providing a current to a coil to generate a magnetic field in the fluid to be monitored, and detecting a voltage, generated across the fluid, which is generally proportional to the product of the magnetic field strength and the flow velocity.
2. Description of Prior Art
A common method of signalling the flow rate is for the flowmeters to control the current flowing in a current signalling loop (also sometimes referred to as a_sensing loop or simply a current loop), the magnitude of the current in the current sensing loop_indicating the rate of fluid flow, the current being arranged to flow in a current signalling_loop incorporating a display or other metering apparatus remote from the sensor probe of the flowmeter. Commonly, a current output in the current signalling loop in the range of 4-20 mA is used; a current of 4 mA indicating zero rate of flow, and 20 mA indicating a full rate of flow, the scale typically being linear.
It has recently been proposed to power an electromagnetic flowmeter from the output current flowing in the current signalling loop. This has the benefit of two-wire operation; the meter power and output signal are conveyed along the same pair of wires. The power constraints in such a flowmeter are very severe, however.
One recent proposal attempts to make powering a meter from a current signalling loop feasible by reducing power consumption at lower flow rates, when the current flowing in the current signalling loop will be smaller, by cycling measurement, and only measuring for short cycles at low flow rates to conserve power. This requires complicated control circuitry, adding to the cost of the flowmeter.
In addition, the above-described proposal is believed to make use of the fact that a typical voltage available in the current signalling loop is about 24V. However, the inventor considers it better practice to provide a flowmeter which will reliably control the current signalling loop with a voltage drop of only about 10V across the flowmeter, to ensure reliable operation when several devices are connection in series in the same current signalling loop. This however would allow only about 40 mW to operate the sensor at low flow rates, and the above described proposal may not function reliably at such low power levels.
This disadvantage is compounded, as the inventor has realised, by the fact that it is particularly when the flow rate is lowest that it is most difficult to obtain reliable measurements with a high signal-to-noise ratio; the signal generated by such a flowmeter is generally proportional to the coil current applied to the coil and to the flow velocity, and at low velocities, the signal will be small for a given coil current.
Thus, where power is limited, it is difficult to obtain a good signal-to-noise ratio at low flow levels. This problem is particularly pronounced in the prior art current signalling loop-powered flowmeter, in which the available power drops as the rate of fluid flow drops. Although practical improvements on the prior art may be achieved, with attendant cost and complexity, by further attempts to reduce power consumption of the elctronics and optimisation of the switching algorithms, the inventor has appreciated that, owing to the problems at low flow rates, the performance of such current signalling loop powered flowmeters is inherently limited. The inventor has proposed a novel alternative which is both simpler to implement and can provide better results, particularly at low flow rates.